Functional and stress testing of LGA devices

ABSTRACT

Improved methods, systems, and apparatuses are disclosed for testing LGA devices. One example embodiment include vertical routing of test nest assembly cooling lines in order to minimize the test nest footprint and increase available test sites on a single test card. Another example embodiment includes isolating and adjusting external loads and moments into the heatsink/cold plate, wherein these loads and moments involve controlling the centroid to restore more ideal thermal performance of the heatsink/chip interface. Still another example embodiment includes a nest architecture facilitating easy and low-cost replacement of LGA sockets. Finally, another example embodiment includes efficient condensation control of test nest assembly parts by using dry-air exhaust.

CROSS REFERENCE

This application is a divisional application of Ser. No. 11/033,935 U.S.Pat. No. 7,352,200 entitled IMPROVED FUNCTIONAL AND STRESS TESTING OFLGA DEVICES, filed Jan. 12, 2005, the disclosure of which isincorporated herein in its entirety for all purposes.

FIELD OF INVENTION

The invention generally relates to functional and stress testing of landgrid array (LGA) devices, and, in particular, to improved apparatuses,methods and systems therefor in relation to a module, which is thedevice under test, i.e., DUT.

BACKGROUND

Prior to installation in machines, manufacturers and assemblers, forinstance, often test processor chips (“chips”) to determine theirperformance metrics. These metrics involve investigating chipperformance using functional and non-functional test sequencesthroughout predetermined windows, e.g., operational, for severalvariables, including chip voltage, clock speed, power and temperature.Although testing various chips' performance metrics readily allows forsorting these chips into their proper class of machine, testing is alsoused to identify chip failures, allow higher machine manufacturingproductivity, and improve product quality.

LGA generically refers to interconnect technology for connecting amodule, i.e., chip, to a board. From a board perspective, the board issaid to have an LGA site or socket. From a module perspective, themodule is said to have an LGA surface. Regardless of which perspective,the electronic connection between the module having the chip and theboard is the same; that is, connection is via a conductive materialcalled an LGA interposer. To make the temporary or permanent connectionbetween the board and module, oftentimes there is a loading mechanism,such as squeezing, wherein the connection results. For testingenvironments, temporary is likely preferred because connection is for alimited time and purpose, i.e., testing. By comparison, connections incommercially available computers are often permanent.

For testing, chips are often attached to a temporary carrier, e.g.,ceramic, and then placed in a device tester for testing. Beforediscussing testers, a brief discussion of chip testing terminology ishelpful. Temporary chip attachment of a single chip is called TCA, and,it follows that the temporary carrier for the TCA is called a TCAcarrier. Alternatively, the chip(s) may not be placed on a temporarycarrier, but be immediately placed on its final production levelcarrier. In this case, the module is referred to as single chip module(SCM) or multichip module (MCM), depending on whether one or more chipsare placed on the carrier. Upon placing the TCA, SCM or MCM into atester, this module is understood to be the device under test, i.e.,DUT. After testing the TCA, the chip is normally sheared from its TCAcarrier, which is normally recycled for use as a carrier for futuretesting, and the chip(s) are permanently mounted on its final carrier.

Testers, themselves, include one or more chip testing components, whichenable determination of chip performance metrics and their ultimatecharacterization. Previous testers include Early Run-In Functional(“ERIF”) 3 and ERIF-4. The tester controls the applied voltage, theclock frequency, test sequences, power, and temperature. Despitediffering architectures, as a whole, testers typically include: 1) atest board that is compatible with the class of DUT; 2) a controlcomputer to control the test sequences; 3) programmable power sources,control hardware to control delivery of test resources, e.g., voltage,current, pneumatics, coolant fluid, etc.; 4) a chiller to delivercoolant fluid to the DUT; 5) network interface and communicationhardware/software; and 6) a test nest, which is the portion of thetester that physically accommodates the DUT.

Despite advances in testing chips, problems and inefficiencies remain inthe testers and methods used for testing modules as the DUTs. Forexample, nests are unnecessarily cumbersome, a problem which results ina disproportionately large nest footprint, and, thereby, seizesotherwise useable and valuable test volume; the cumbersome nestarchitecture also lends itself to unnecessary difficulties ininstallation and replacement of test sockets. In addition, testers oftenhave external parasitic loads and moments acting on the heatsink andchip surface without having an ability to isolate and/or adjust theseloads and moments; as a result, deviations from the chip's trueperformance metrics, especially those related to temperature, are likelyto occur for the DUT. Another problem in the current state of the art isthe effect of inattentiveness to controlling condensation on the DUT,wherein, condensation may cause damage to the tester.

In light of the example, above-identified problems, a need, therefore,exists, for improved apparatuses, methods and systems for testingmodules as the DUT.

SUMMARY OF THE INVENTION

Embodiments of the invention generally provide methods, systems, andapparatuses pertaining to improved testing of LGA modules.

Improved methods, systems, and apparatuses generally include routingcoolant lines to and from a test nest assembly in substantially verticalorientations. Such an orientation minimizes the nest footprint, and,thereby, enables test nests in a confined test volume and multiple testsites on a single test card within a test nest.

In addition, improved methods, systems, and apparatuses generallyinclude isolating and adjusting external loads and moments transmittedto a heatsink during the testing of a module. Isolating and adjusting tothe point of removal of these loads and moments assists in obtainingideal thermal performance at the interface of the heatsink and the DUT.As a result, more ideal conditions allow for truer determinations ofchip performance metrics—a primary reason for engaging in the chiptesting.

Improved methods, systems, and apparatuses also generally includeproviding a test nest architecture that enables facile installation andreplacement of low-cost LGA test sockets.

Additionally, improved methods, systems, and apparatuses also generallyinclude improved and efficient condensation control of the test nestassembly parts through use of a secondary condensation containmentchamber. The secondary chamber at least partially borders the primarychamber, which provides condensation control to the heatsink andotherwise unused dry-air exhaust to the secondary chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained and can be understood indetail, a more particular description of the invention, brieflysummarized above, may be had by reference to the embodiments thereofwhich are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 depicts a test nest assembly in accordance with the disclosedinvention.

FIG. 2 depicts a detailed view of test nest assembly in accordance withthe disclosed invention.

FIG. 3 depicts a detailed view of external load isolation and centroidadjustment hardware in accordance with the disclosed invention.

FIG. 4 depicts an exploded view of base nest assembly and backsidehardware in accordance with the disclosed invention.

FIG. 5 depicts a test nest assembly showing secondary condensationcontrol chamber in accordance with the disclosed invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The following is a detailed description of example embodiments of theinvention depicted with assistance and reference to the accompanyingdrawings. The embodiments are examples and are in such detail as toclearly communicate the invention. However, the amount of detail offeredis not intended to limit the anticipated variations of embodiments; onthe contrary, the intention is to cover all modifications, equivalents,and alternatives falling within the spirit and scope of the presentinvention as defined by the appended claims. The detailed descriptionsbelow are designed to make such embodiments obvious to a person ofordinary skill in the art.

Generally speaking, improved systems, methods, and apparatuses fortesting modules, e.g., TCA, SCM, and MCM, in an LGA socket within atester's nest section are contemplated. As a preliminary matter,although this disclosure discusses exposed chip LGA devices, it isunderstood that this disclosure equally applies, extends, and enablestesting of lidded chip LGA device.

One example embodiment includes vertical routing of test nest assemblycooling lines in order to minimize the test nest footprint and increaseavailable test sites on a single test card. Another example embodimentincludes isolating and adjusting external loads and moments into theheatsink (also referred to as “cold plate”), wherein these loads andmoments involve controlling the centroid to restore more ideal thermalperformance of the heatsink/chip interface. Still another exampleembodiment includes a nest architecture facilitating easy and low-costreplacement of LGA sockets. Finally, another example embodiment includesefficient condensation control of test nest assembly parts by usingdry-air exhaust.

Referring to FIG. 1, an example, simplified view of the test nestassembly 100 is presented. The test nest assembly 100 includes an aircylinder 105 that controls the engagement of a heatsink assembly 110,onto the exposed chip of the module. The heatsink assembly 110accommodates a flow of chilled fluid, e.g., Dynalene®, to facilitateheat exchange between the module and the heatsink assembly 110. Theheatsink assembly 110 is optionally gimbaled in order to align to thetop surface of the exposed chip and create a uniform thermal path, and,thereby, enhance the ability to calculate a chip's true performancemetrics. The air cylinder 105 also controls a load engagement frame thatis part of the heatsink assembly that loads the module around itsperimeter to actuate the LGA socket, which electrically interconnectsthe module to the test board 125 mounted to a base plate 130. In theoperating position, the test nest is “down,” which means the heatsink110 is engaged onto the module. The air cylinder 105 can also bepositioned in the “up” position to facilitate module insertion andremoval. As the test nest assembly 100 traverses from the “up” and“down” positions, the coolant lines 140 routing and support structuremoves with the double-ended air cylinder 105.

Turning now to FIG. 2, an example, detail view of the test nest assembly200 is depicted on a base plate 203 having a test board 205. Theheatsink assembly 207, nest side frames 210, and non-depicted primarycontainment chamber are all mounted to the suspension plate 215, whichis normally rigidly attached to the moving shaft of the air cylinder220. The air cylinder 220, in this example embodiment, is double-ended,and the coolant line carrier plate 225 is rigidly attached to the topmoving shaft of the air cylinder 220. The load isolation block 230 andcentroid adjustment plate 235 are also mounted on the coolant linecarrier plate 225, wherein each load isolation block 230 is topped byupper coolant fitting 233. As the air cylinder 220 moves to theoperational position, there is a slight, i.e., approximately 1 mm,relative motion between the heatsink 207 and the coolant line carrierplate 225 as the heatsink 207 displaces from its rest position into asuspended (i.e., on suspension springs) operational position. Thisrelative motion must be accommodated to prevent significant compressiveloads from developing in the coolant lines, each of which comprises arigid coolant line 240 coupled 245 to a flexible coolant line 250. Suchhigh compressive loads could also attribute to centroid deviation ifthey are not adequately relieved.

FIGS. 1 and 2 provide visual aid in the coolant lines' architecture,which is substantially vertical in direction as compared to the heatsinkassembly having the heat sink. Verticality of the coolant linesminimizes the nest footprint, especially as compared to the ERIF-4routing, which uses horizontal hose routing close to the board, i.e., alarge footprint. In short, a minimized footprint enables test nests in aconfined test volume and multiple test sites on a single test card.

Moving on to FIG. 3, this figure shows additional hardware for the testnest assembly 300 shown in FIGS. 1 and 2, wherein this additionalhardware 300 accommodates the relative motion of the heatsink and thecarrier plate 305. The coolant lines routing from the heatsink terminateon a load isolation block 310. The load isolation block 310 may be madeof Ultem® or similar low thermal conductivity material to inhibitcondensation on its outer surfaces. The coolant lines are routed throughthe load isolation block 310 with a bulkhead fitting 315, wherein thecoolant lines attach to lower 320 and upper 325 coolant fittings. Theload isolation block 310 is restrained laterally by two pins 330, 331that are press fit into the centroid adjustment plate 335, which isnormally rigidly attached to a coolant line carrier plate 305 withscrews for instance. These pins 330, 331 engage a hole and slot in theload isolation block 310 to restrict lateral motion. However, the loadisolation block 310 is free to move vertically with respect to thecoolant line carrier plate 335. This motion is facilitated by a linearball bearing 340 that is installed in the load isolation block 310 andwhich engages one of the guide pins 330, 331. As the heatsink engagesthe module under test and moves vertically with respect to thesuspension plates, this relative motion is transmitted to the loadisolation blocks 310, which also respond vertically, hence eliminatingany significant compressive load development in the coolant lines.

The thermal performance of the heatsink/chip interface is enhanced by anest design controlling suspension forces and external forces acting onthe heatsink so that the resultant contact force between the two isclose to the geometric center—the closer the better. As the contactforce deviates from true center, the contract pressure distributionbetween the two components will become more variable, and the thermalinterface performance will degrade. Contact force centroid control isachieved primarily by using a low pivot point on the heatsink, by usinglow stiffness compression springs in the suspension, and by making thenominal preload compression of the springs large with respect to thedeviations that would occur due to heatsink rotation duringheatsink/chip surface alignment. Additionally, the external loads actingon the heatsink, primarily those relating from coolant hoses, aregeometrically symmetrical, which contributes to good contact forcecentroid control.

Other forces, which are termed “parasitic forces” in this disclosure,can be transmitted into the heatsink, and these parasitic forces disrupta nominally adjusted centroid, such as the one expectedly coming off thevertical architecture of the coolant lines in FIGS. 1 and 2. Parasiticforces can come from various sources to create external loads andmoments. The sources include heater wires that feed into the heatsink,tubing that delivers helium to the heatsink/chip interface, insulationon the coolant lines where they enter the primary condensation controlchamber, the bulk weight of the coolant lines and insulation beyondwhere they exit the upper coolant fittings, variations in the heatsinksuspension spring loads, and a variety of other sources. Regardless ofthe source, these parasitic forces and moments can be nullified byapplying a compensating force to the heatsink, and specifically bylaterally positioning the centroid adjustment plate 335 with respect tothe coolant line carrier plate 305. This translational motion developsan elastic force in the flexible coolant line 305 that connects thebulkhead fitting 315 to the rigid coolant line 240 as shown in FIG. 2.The length of the flexible coolant line 250 was selected to achieve acentroid shift of approximately 8 mm as both of the centroid adjustmentplates 335 are shifted between their extremes allowed by the clearanceholes and the retention bolts. The centroid is measured using a tripodload cell arrangement that mimics a module. The deviation in the loadsreported by each of the load cells in the tripod can be used, along withthe geometric positioning of each load cell, to compute the position ofthe centroid. The adjustment procedure is first to loosen the adjustmentbolts 350 on both centroid adjustment plates 335, position theadjustment plates 335 until the tripod load cell reports an acceptablecentroid location, and then tighten the adjustment bolts 350.

In addition to the foregoing, lateral forces and moments applied to theupper coolant fittings 325 due to the coolant hose weight or routing arenot transmitted from the coolant lines connected to the lower fittings320 because the linear bearing 340 absorbs these forces and moments.This absorbing results in a more stable centroid location over time.

Now, turning to FIG. 4, an exploded view of the base nest assembly 400is displayed. The base nest assembly 400 includes a base plate 403,which provides the structural support for the air cylinder andassociated hardware. The base plate 403 is mounted to the test board 405and is bolted to a backside stiffener 410 with screws 415, whichprovides the structural rigidity necessary to support the loads requiredto actuate LGA sockets 420. In addition, the back side stiffener 410 mayalso contain heater cartridges 425 retained by cartridge retainers 430,wherein the cartridges 425 warm the backside hardware to mitigatecondensation.

The base assembly 400 also contains commercially available LGA sockets420 with minor modifications to minimize their footprints, wherein thesockets 420 may be easily installed and removed from the test nest.Using low-cost LGA sockets 420 almost presuppose conditions forfacilitating easy and definite need for replacing these sockets 420.These sockets 420 are retained with a socket retainer 435, which ispositioned above the socket 420 to retain it in the test board 405. Thesocket retainer 435 is held in place by two screws on its diagonal,which facilitates its ease of removal.

Turning now to FIG. 5, this figure depicts the secondary condensationcontrol chamber 505 of the test nest assembly structural hardware 500.The secondary chamber 505 is composed of a polymer wall that is fittedclosely to the planar/printed circuit board 510 at its bottom edge andsurrounds both the mechanical nest components 520, 525 and the primarycondensation control containment chamber. Large air moving deviceswithin the test unit chassis can quickly inject high moisture air intothe nest vicinity. The secondary chamber 505 polymer wall acts as abarrier between inner and outer air regions. Within this barrier,components such as the base plate, side plates, and top plate supportingthe air cylinder 525 each have the potential to cause condensation ofwater vapor due to the low temperatures driven by the roughly −20° C.coolant fluid, e.g., Dynalene®, used in the heatsink 520. Thiscondensation is prevented by low-dew-point compressed air introducedinto the primary containment area, which exhausts into the secondarycontainment area. The dry air pathway and condensation control ismanaged, for example, as follows: compressed air at dew pointtemperature of approximately −35° C. and dry bulb temperature of roughly12-18° C. is injected into the primary containment chamber to controlheatsink condensation. Within the primary chamber, the dry bulbtemperature of the compressed air is depressed through convective heattransfer to the heatsink 520, and it exhausts from the primary chamberwhere the air both cools portions of the nest assembly's hardware andprovides low dew point temperature air to the inner air region withinthe secondary condensation control chamber. The dry air mixing with theair in the inner air region lowers the inner air region dew pointtemperature. The outer air region outside of the secondary condensationcontrol chamber has a maximum dew point of 13° C. Surface temperaturesof portions of the nest structural hardware are below this temperature,which would cause water vapor condensation on them if they were notlocated within the dry inner air region. However, the dew point of theinner air region is much lower than the temperature of these hardwarecomponents, and condensation is prevented.

Condensation control is maintained in all tester components with minimumutilization of dry compressed air. If only the secondary containmentcondensation chamber 505 is used, and, hence, the primary chamber iseliminated, a significantly larger compressed air volumetric flow ratewould be needed to prevent condensation on the heatsink 520 and the testnest assembly structural hardware 500. Compressed air is expensive toproduce, and the increased dry air capacity required would increase theoperating cost of the tester. Use of a primary containment chamberfocused on the heatsink condensation control and a secondary chamberfocused on the structural hardware is much more efficient andcost-effective use of compressed air resources and facilities.

While the foregoing is directed to example embodiments of the disclosedinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method for controlling external loads and moments in a heatsink during testing of a chip in a test nest, the method comprising: receiving the external loads and moments from a source associated with the chip during the testing; and isolating the external loads and moments by absorbing both lateral loads and multi-directional moments while concurrently allowing unrestrained vertical motion.
 2. The method of claim 1, further comprising adjusting, after the isolating, the external loads and moments with an elastic reaction force.
 3. The method of claim 1, further comprising eliminating, after the isolating, the external loads and moments with an elastic reaction force. 